244 lines
9.7 KiB
ReStructuredText
244 lines
9.7 KiB
ReStructuredText
.. _scheduling_v2:
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Scheduling
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##########
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The kernel's priority-based scheduler allows an application's threads
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to share the CPU.
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.. contents::
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:local:
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:depth: 2
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Concepts
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********
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The scheduler determines which thread is allowed to execute
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at any point in time; this thread is known as the **current thread**.
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Whenever the scheduler changes the identity of the current thread,
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or when execution of the current thread is supplanted by an ISR,
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the kernel first saves the current thread's CPU register values.
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These register values get restored when the thread later resumes execution.
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Thread States
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=============
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A thread that has no factors that prevent its execution is deemed
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to be **ready**, and is eligible to be selected as the current thread.
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A thread that has one or more factors that prevent its execution
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is deemed to be **unready**, and cannot be selected as the current thread.
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The following factors make a thread unready:
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* The thread has not been started.
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* The thread is waiting on for a kernel object to complete an operation.
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(For example, the thread is taking a semaphore that is unavailable.)
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* The thread is waiting for a timeout to occur.
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* The thread has been suspended.
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* The thread has terminated or aborted.
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Thread Priorities
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=================
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A thread's priority is an integer value, and can be either negative or
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non-negative.
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Numerically lower priorities takes precedence over numerically higher values.
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For example, the scheduler gives thread A of priority 4 *higher* priority
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over thread B of priority 7; likewise thread C of priority -2 has higher
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priority than both thread A and thread B.
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The scheduler distinguishes between two classes of threads,
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based on each thread's priority.
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* A :dfn:`cooperative thread` has a negative priority value.
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Once it becomes the current thread, a cooperative thread remains
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the current thread until it performs an action that makes it unready.
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* A :dfn:`preemptible thread` has a non-negative priority value.
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Once it becomes the current thread, a preemptible thread may be supplanted
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at any time if a cooperative thread, or a preemptible thread of higher
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or equal priority, becomes ready.
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A thread's initial priority value can be altered up or down after the thread
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has been started. Thus it possible for a preemptible thread to become
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a cooperative thread, and vice versa, by changing its priority.
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The kernel supports a virtually unlimited number of thread priority levels.
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The configuration options :option:`CONFIG_NUM_COOP_PRIORITIES` and
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:option:`CONFIG_NUM_PREEMPT_PRIORITIES` specify the number of priority
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levels for each class of thread, resulting the following usable priority
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ranges:
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* cooperative threads: (-:option:`CONFIG_NUM_COOP_PRIORITIES`) to -1
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* preemptive threads: 0 to (:option:`CONFIG_NUM_PREEMPT_PRIORITIES` - 1)
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For example, configuring 5 cooperative priorities and 10 preemptive priorities
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results in the ranges -5 to -1 and 0 to 9, respectively.
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Scheduling Algorithm
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====================
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The kernel's scheduler selects the highest priority ready thread
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to be the current thread. When multiple ready threads of the same priority
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exist, the scheduler chooses the one that has been waiting longest.
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.. note::
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Execution of ISRs takes precedence over thread execution,
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so the execution of the current thread may be supplanted by an ISR
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at any time unless interrupts have been masked. This applies to both
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cooperative threads and preemptive threads.
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Cooperative Time Slicing
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========================
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Once a cooperative thread becomes the current thread, it remains
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the current thread until it performs an action that makes it unready.
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Consequently, if a cooperative thread performs lengthy computations,
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it may cause an unacceptable delay in the scheduling of other threads,
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including those of higher priority and equal priority.
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To overcome such problems, a cooperative thread can voluntarily relinquish
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the CPU from time to time to permit other threads to execute.
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A thread can relinquish the CPU in two ways:
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* Calling :cpp:func:`k_yield()` puts the thread at the back of the scheduler's
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prioritized list of ready threads, and then invokes the scheduler.
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All ready threads whose priority is higher or equal to that of the
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yielding thread are then allowed to execute before the yielding thread is
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rescheduled. If no such ready threads exist, the scheduler immediately
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reschedules the yielding thread without context switching.
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* Calling :cpp:func:`k_sleep()` makes the thread unready for a specified
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time period. Ready threads of *all* priorities are then allowed to execute;
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however, there is no guarantee that threads whose priority is lower
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than that of the sleeping thread will actually be scheduled before
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the sleeping thread becomes ready once again.
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Preemptive Time Slicing
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=======================
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Once a preemptive thread becomes the current thread, it remains
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the current thread until a higher priority thread becomes ready,
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or until the thread performs an action that makes it unready.
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Consequently, if a preemptive thread performs lengthy computations,
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it may cause an unacceptable delay in the scheduling of other threads,
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including those of equal priority.
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To overcome such problems, a preemptive thread can perform cooperative
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time slicing (as described above), or the scheduler's time slicing capability
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can be used to allow other threads of the same priority to execute.
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The scheduler divides time into a series of **time slices**, where slices
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are measured in system clock ticks. The time slice size is configurable,
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but this size can be changed while the application is running.
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At the end of every time slice, the scheduler checks to see if the current
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thread is preemptible and, if so, implicitly invokes :cpp:func:`k_yield()`
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on behalf of the thread. This gives other ready threads of the same priority
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the opportunity to execute before the current thread is scheduled again.
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If no threads of equal priority are ready, the current thread remains
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the current thread.
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Threads with a priority higher than specified limit are exempt from preemptive
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time slicing, and are never preempted by a thread of equal priority.
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This allows an application to use preemptive time slicing
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only when dealing with lower priority threads that are less time-sensitive.
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.. note::
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The kernel's time slicing algorithm does *not* ensure that a set
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of equal-priority threads receive an equitable amount of CPU time,
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since it does not measure the amount of time a thread actually gets to
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execute. For example, a thread may become the current thread just before
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the end of a time slice and then immediately have to yield the CPU.
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However, the algorithm *does* ensure that a thread never executes
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for longer than a single time slice without being required to yield.
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Scheduler Locking
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=================
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A preemptible thread that does not wish to be preempted while performing
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a critical operation can instruct the scheduler to temporarily treat it
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as a cooperative thread by calling :cpp:func:`k_sched_lock()`. This prevents
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other threads from interfering while the critical operation is being performed.
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Once the critical operation is complete the preemptible thread must call
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:cpp:func:`k_sched_unlock()` to restore its normal, preemptible status.
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If a thread calls :cpp:func:`k_sched_lock()` and subsequently performs an
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action that makes it unready, the scheduler will switch the locking thread out
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and allow other threads to execute. When the locking thread again
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becomes the current thread, its non-preemptible status is maintained.
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.. note::
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Locking out the scheduler is a more efficient way for a preemptible thread
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to inhibit preemption than changing its priority level to a negative value.
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.. _thread_sleeping:
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Thread Sleeping
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===============
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A thread can call :cpp:func:`k_sleep()` to delay its processing
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for a specified time period. During the time the thread is sleeping
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the CPU is relinquished to allow other ready threads to execute.
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Once the specified delay has elapsed the thread becomes ready
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and is eligible to be scheduled once again.
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A sleeping thread can be woken up prematurely by another thread using
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:cpp:func:`k_wakeup()`. This technique can sometimes be used
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to permit the secondary thread to signal the sleeping thread
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that something has occurred *without* requiring the threads
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to define a kernel synchronization object, such as a semaphore.
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Waking up a thread that is not sleeping is allowed, but has no effect.
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.. _busy_waiting:
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Busy Waiting
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============
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A thread can call :cpp:func:`k_busy_wait()` to perform a ``busy wait``
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that delays its processing for a specified time period
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*without* relinquishing the CPU to another ready thread.
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A busy wait is typically used instead of thread sleeping
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when the required delay is too short to warrant having the scheduler
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context switch from the current thread to another thread and then back again.
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Suggested Uses
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**************
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Use cooperative threads for device drivers and other performance-critical work.
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Use cooperative threads to implement mutually exclusion without the need
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for a kernel object, such as a mutex.
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Use preemptive threads to give priority to time-sensitive processing
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over less time-sensitive processing.
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Configuration Options
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*********************
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Related configuration options:
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* :option:`CONFIG_NUM_COOP_PRIORITIES`
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* :option:`CONFIG_NUM_PREEMPT_PRIORITIES`
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* :option:`CONFIG_TIMESLICING`
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* :option:`CONFIG_TIMESLICE_SIZE`
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* :option:`CONFIG_TIMESLICE_PRIORITY`
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APIs
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****
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The following thread scheduling-related APIs are provided by :file:`kernel.h`:
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* :cpp:func:`k_current_get()`
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* :cpp:func:`k_sched_lock()`
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* :cpp:func:`k_sched_unlock()`
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* :cpp:func:`k_yield()`
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* :cpp:func:`k_sleep()`
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* :cpp:func:`k_wakeup()`
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* :cpp:func:`k_busy_wait()`
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* :cpp:func:`k_sched_time_slice_set()`
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